A cirque is a small glacial valley, originally home to a small feeder glacier. Once the glacier melts, it leaves a small, high valley, often described as 'bowl-shaped' or 'amphitheater-like'. They may be very well defined and almost crater-like, or they may be very uneven and quite hard to pick out if you don't know what you're looking for. They may be up to a square kilometer in size, but are often smaller.
A cirque is formed when snow starts to build on the side of a mountain. Snow is most likely to accumulate on the leeward and/or shadowed slopes of mountains, but in sufficiently cold climates cirques can form on any slope, and many peaks are surrounded by two cirques (forming an arête) or three (forming a horn). Once the snow forms a ice pack, either through nivation or through pressure from the weight of the ice, it can start to carve out a small valley.
The valley is formed through a number of processes, including:
- Freeze/thaw erosion: freezing ice expands in cracks in the rock, causing them break. They are then carried away as the ice flows downhill.
- Plucking: when the ice moves down the mountain it pulls away from the mountainside, leaving a space where water can enter and freeze. This new ice freezes around rocks and protuberances on the mountainside, and when the glacier continues to move downhill it rips these rocks from the mountain (in extreme cases this may result in a rock glacier).
- Abrasion: once the glacier is big enough, the shear force of its weight moving over the ground will pull rocks from the bottom and sides of the cirque.
Cirques are often found at glacier heads (the 'upstream' ends glaciers), or along the path of a glacier. The smaller icepacks of the cirque will feed into the larger glacier. Because the ice flows into the surface of the glacier, a cirque will only erode down to the top of the main glacier, and not down to the level of the main valley floor. When the ice melts, the cirque is left as a small hanging valley, with high cliffs on three sides, and an open side where it emptied out into the main valley.
Some other cirque related terms: A tarn is a lake within the bowl of a cirque. When two cirques form on opposite sides of a peak, they form an arête; if the peak between these starts to erode it forms a col. When a peak is surrounded on three or more sides by cirques, it forms a sharp spike, which may be called a pyramidal peak or a glacial horn, usually shortened to peak or horn.
AKA coomb, coombe, and cwm.